Biomedical subjects

L Bowns

Publications and source records attributed to L Bowns.

5 recordsLinked to original sources

PubMed2002

Can spatio-temporal energy models of motion predict feature motion?

Current "spatio-temporal energy" models of how we perceive pattern motion have been very successful in helping us to understand the mechanisms of motion perception. Although they have been supported by a large number of physiological and psychological studies, they have so far not provided a complete explanation for a number of results. These results emerge from experiments concerned with predicting perceived motion direction from patterns comprising two or more components. It has been suggested that these results are more consistent with an earlier type of model based on the motion of two-dimensional features. This paper briefly describes how three generic spatio-temporal energy models have been extended to predict motion derived from two-component stimuli. A new model is then presented that utilises similar architecture to the two-stage spatial-temporal energy model proposed by Adelson and Movshon (Nature 300 (1982) 523). The first stage is a spatial temporal filtering stage and the second stage computes the intersection of constraints (IOC), an important constraint used in combining motion information across two or more components. In the model presented here the second stage is different. A directional spatial second derivative is used to extract zero-crossings at the component level, i.e. gratings. If any zero-crossing falls in the same spatial position for two or more components its displacement is tracked using a nearest neighbour match. Tracking these 'intersecting zero-crossings' essentially computes the IOC but also provides other properties that predict non-IOC motion, and second-order component motion. Surprising new insights are described into how current spatio-temporal energy models may also account for these results. However, unlike the model presented here, they rely on operations carried out on the two-dimensional pattern.

Contrast Sensitivity↗

PubMed2001

IOC, vector sum, and squaring: three different motion effects or one?

Bowns (Vision Research, 36(22) (1996), 3685) argued that there are distinct features in two-component moving patterns (plaids) that if tracked move in the same direction as (1) the intersection of constraints direction (IOC) Adelson and Movshon (Nature, 300 (1992), 523); and (2) the vector sum direction (VS) Yo and Wilson (Vision Research, 32(1) (1992), 135). The IOC and VS are hypotheses of how the motion of single components is combined to give pattern motion. This paper shows that there are also features that provide an explanation for a reversed motion described by Derrington, Badcock, and Holroyd (Vision Research, 32(4), (1992), 699), and investigates why reversals only occur under specific conditions. Section 3 replicates the original study by Derrington et al. (1992) and confirms that the reversals are limited to low temporal frequencies. Section 4 varies the spatial displacement of features that also predict reversals and shows that the temporal frequency at which reversals occur varies and is linearly dependent on the displacement of these specified features. Derrington et al. (1992) showed that reversals only occur when components have oblique angles, and suggested an explanation in terms of speed differences. Section 5 was not consistent with this hypothesis. An alternative explanation for why reversals only occur at oblique angles, and at low spatial frequencies is provided in terms of feature properties. Results supporting the IOC, vector sum, and squaring have previously been interpreted in terms of three disparate mechanisms. This may not be necessary.

Discrimination, Psychological↗

PubMed2001

Features derived from first-order motion mechanisms predict anomalies in motion perception.

Current dominant hypotheses of how humans detect the movement of patterns assume that the pattern is divided into one-dimensional sinusoidally varying luminance patterns, referred to as gratings (first-order components). The speed of these gratings is independently encoded from predominantly spatial and temporal frequency information, and their direction is encoded from orientation information. This paper addresses the problem of how the individually encoded grating information is combined to give perceived pattern direction, given that real moving objects are generally made up of more than one component. More specifically, further evidence is presented for a combination based on the use of a feature derived from first-order components--'first-order feature hypothesis'. This hypothesis essentially implements a constraint on pattern direction called the intersection of constraints (IOC) proposed by Adelson and Movshon [1982, Nature 300 523-525]. A simulation of the model is used to make three new predictions about a perceived motion reversal reported by Derrington et al (1992, Vision Research 32 699-707); these predictions are tested and found to be consistent with the first-order feature hypothesis.

Computer Simulation↗

PubMed1996

Evidence for a feature tracking explanation of why type II plaids move in the vector sum direction at short durations.

When two moving sinusoidal gratings, with similar spatial frequency, contrast, phase, but different orientation are combined to form a plaid, their perceived direction of motion has been predicted by the intersection of constraints rule (IOC) (Adelson & Movshon, Nature, 300, 523-525, 1982). However, at short durations (60 msec) the direction of perceived motion has been predicted by the vector sum direction for "Type II" plaids (Yo & Wilson, Vision Research, 32, 1, 1992). Type II plaids are the set of plaids where the components are both located on one side of the resultant computed using the IOC rule. Yo and Wilson suggest that the vector sum direction is observed for Type II plaids at short durations because non-Fourier information is not available and direction is computed from Fourier information only. The first experiment in this study replicates the original Yo and Wilson result using similar stimuli but a simpler task; perceived direction was measured using a direction discrimination task instead of the method of adjustment used by Yo and Wilson. The second experiment provides evidence against generalizing the result to all Type II plaids. A systematic set of type II plaids that varied only in terms of the orientation of the second component provided an ideal set because their predicted motion direction followed very different patterns when predicted by the IOC and vector sum computations. The results obtained were predicted more accurately by the IOC than the vector sum. Experiment 3 provides further evidence that movement in the vector sum direction is not a general property of type II plaids. A small change to the velocity of one of the components of a plaid previously perceived in the vector sum direction had the effect of shifting the perceived motion in the IOC direction, despite increasing the difference between the IOC and VS predictions. This result is not consistent with Yo and Wilson's hypothesis that Type II plaids move in the vector sum direction because of a temporal delay between Fourier and non-Fourier information. Computational analysis of the stimuli used in both the current and original experiments revealed a possible explanation of the results in terms of a contribution from local feature tracking rather than a vector sum operation.

Fourier Analysis↗

PubMed1994

Hemifield relative motion bias in adults monocularly enucleated at an early age.

A psychophysical study of relative motion discrimination for six subjects monocularly enucleated within 24 months of birth showed no significant difference in their thresholds for detecting relative velocity when compared with age matched control subjects. The study however highlighted a bias for the control group, age matched normals, which is consistent with a hemifield anisotropy for motion in normal observers reported by Smith and Hammond [(1986) Perception, 15, 111-117]. The bias however was found to be reversed for enucleates. This difference and individual differences reported in the Smith and Hammond study are discussed in terms of possible developmental changes which emerge when stereopsis is absent or weak.

Adult↗